Along with recent demands for smaller and lower profile electronic devices, finer semiconductor devices to be mounted onto these electronic devices have been increasingly demanded. For example, the design rule for mask patterns has required that an image with a size of a line and space (“L & S”) of less than 0.1 μm be extensively formed and it is expected to require circuit patterns of less than 80 nm in the near future. The L & S denotes an image projected onto a wafer in exposure with equal line and space widths, and serves as an index of exposure resolution. The exposure has three important factors including resolution, overlay accuracy, and throughput. The resolution is the minimum size for a precise pattern transfer. The overlay accuracy is a precision with which to overlay multiple patterns over an object to be exposed. The throughput is the number of sheets exposed per unit of time.
A projection exposure apparatus as a typical exposure apparatus for manufacturing semiconductor devices includes a projection optical system for exposing a pattern formed on a mask (reticle) onto a wafer, and its resolution R is given by Rayleigh's equation below:R=k1(λ/NA)  (1)where λ is a wavelength of a light source, NA is a numerical aperture of the projection optical system, kl is a constant determined by a development process and others.
As the shorter the wavelength becomes and the higher the NA increases, the better the resolution becomes. The recent trend has required that the resolution be a smaller value; however it is difficult to meet this requirement using only the increased NA, and the improved resolution expects use of a shortened wavelength. Exposure light sources have currently been in transition from KrF excimer laser (with a wavelength of approximately 248 nm) and ArF excimer laser (with a wavelength of approximately 193 nm) to F2 excimer laser (with a wavelength of approximately 157 nm). Practical use of the EUV light is being promoted as a light source.
An exposure apparatus using the EUV as a light source (referred to as an “EUV exposure apparatus” hereinafter) is used for exposure of a circuit pattern of 100 nm or less, and the permissible overlay error is strict in the exposure process for such a fine pattern. Therefore, the overlay accuracy should be higher than the conventional one and reduces the overlay error caused by such a deformed optical element as a mirror due to environmental changes, such as varying temperature, and exposure heat. For example, Japanese Patent Application Publication No. 11-219900 proposes an exposure apparatus and method that improves the overlay accuracy through driving control of a reticle in the optical-axis direction, and corrects an exposure transfer magnification and positional offsets.
The driving control of the reticle proposed by Japanese Patent Application Publication No. 11-219900 may correct the transfer magnification only in a longitudinal direction of a slit or a direction orthogonal to a scan direction, and thus cannot always provide the EUV exposure apparatus with satisfactory overlay accuracy. In addition, unavailable independent corrections of positional offsets in the longitudinal and lateral (or scan) directions of the slit are insufficient to improve the overlay accuracy.